This application claims the priority of DE 198 25 305.2, filed Jun. 5, 1998, the disclosure of which is expressly incorporated by reference herein.
The invention relates to a method for correcting the airflow in an internal combustion engine, drawn in through an induction pipe and measured in the induction pipe.
A method of this kind is known for example from German Patent document DE 33 44 276 C2. In this known method, the volume of air drawn into the internal combustion engine is measured using a thermal flow meter. A differential of the volume of air drawn in is determined on the basis of a changing property of the volume of air drawn in. The differential represents a measure of the magnitude of the oscillations of the drawn-in air. A correction is performed on the basis of this differential. However, such a correction by means of the differential reacts in an undesirably sensitive manner to even brief pulsations and/or changes in the oscillation of the amplitude.
To establish the need for a method for correcting the airflow drawn in through an induction pipe and measured in the induction pipe in an internal combustion engine, the technical area is described below.
In engine control devices for internal combustion engines, the drawn-in airflow is measured by a special sensor, for example a hot-film airflow meter. For this purpose, the output voltage of the sensor is sampled in a time-synchronized manner and converted to a corresponding airflow value by characteristic interpretation. Then, the calculated values are added together. Finally, in synchronization with the crankshaft (for example, all 180.degree. crankshafts in four-cylinder internal combustion engines) the arithmetic average is formed. This average is also termed the load signal and is used, in particular, for determining the ignition point and for calculating the injection time, for example for achieving a stoichiometric combustion.
Depending on the type of engine and the induction pipe geometry, at certain operating points oscillations of the air column in the induction pipe (pulsations) develop. These oscillations cause incorrect measurements at the sensor located in the induction pipe (undesired measurement of the reflow). Minor oscillations can be eliminated by forming the average, in other words generating the average provides a correct result. The reflows, however, can become so large that the measured airflow no longer corresponds to the true average. As a result, an incorrectly calculated injection time results and hence non-ideal combustion that leads to deteriorated exhaust behavior. The A/F control used in vehicles equipped with catalytic converters and with the oxygen sensor located in the exhaust line can compensate only partially for this improper adjustment since the incorrect measurements can exceed its regulating range.
The goal of the present invention is to detect these pulsations and to correct the measured average. In addition, the correction is made by a method that is not sensitive to interference.
This goal is achieved by a method for correcting the airflow in an internal combustion engine dawn in through an induction pipe and measured in the induction pipe, characterized in that the measured airflow (U or m.sub.LM) is multiplied by a correction factor (KS) that is formed as a function of a value expressing the degree of the measurement error .sigma..sup.2 and a value expressing the direction of the measurement error h.sub.v.
Advantageous improvements are described herein.
According to the invention, the measured airflow, especially in the form of the measured average, is multiplied by a correction factor that is formed as a function of a value expressing the degree of the measurement error and a value expressing the direction of the measurement error.
Preferably, the value expressing the degree of the measurement error is determined by means of a statistical calculation method. Calculation of the known standard deviation .sigma. or the variance .sigma..sup.2 is suitable for this purpose. The value expressing the degree of the measurement error corresponds to the degree of pulsations of the oscillating airflow in the induction pipe. To calculate the standard deviation, the sum of the input voltages and the sum of the squares of the input voltages of a sensor (for example a hot-film airflow meter) must be calculated. The standard deviation is determined on a segment-synchronized basis corresponding to averaging in calculating the airflow. The value expressing the degree of the measurement error (for example the standard deviation or variance) can be used directly for correcting the measured airflow, with the measured and preferably averaged value of the airflow being multiplied by a correction factor that is preferably obtained from a characteristic, a characteristic map, or even from a characteristic space. The value expressing the degree of the measurement error provides only information on the amount by which the measured airflow is corrected, but there is no information on whether the correction should involve increasing or reducing the measured airflow.
This second piece of information is supplied by the value expressing the direction of the measurement error. The value expressing the direction of the measurement error is determined by at least one operating value of the internal combustion engine, for example the engine rpm and/or the throttle position and/or the valve lift in internal combustion engines with lift adjustment of the intake valves. For this purpose, operating values are used in particular by which an anticipated airflow in the induction pipe can be estimated from the resultant operating state of the internal combustion engine. For example, the estimated anticipated airflow is compared with the measured airflow in the form of a plausibility monitoring function. If the measured airflow is higher than the anticipated estimated airflow because of the available operating value, the value expressing the direction of the measurement error provides the information that the measured airflow must be reduced for correction. However, if the measured airflow is less than the airflow to be anticipated on the basis of the available operating value, the value expressing the direction of the measurement error provides the information that the measured airflow must be increased during correction. The operating value itself can be the value expressing the direction of the measurement error.
Preferably, the correction factor is determined by use of a characteristic map whose first input value is the value expressing the degree of the measurement error and whose second input value is the value expressing the direction of the measurement error.
With this method according to the invention, a highly accurate and interference-insensitive correction of the measured airflow in the induction pipe is possible.
Other objects, advantages and novel features of the present invention will become apparent from the following detailed description of the invention when considered in conjunction with the accompanying drawings.